Microsoft has a very informative post on the pros and cons of 64 bit vs. 32 bit performance on SQL Server 2005. This post confirms and reinforces many of the results that I have seen running a VLDB on 64 bit SQL Server 2005.
Specifically, with 64 bit, it is very important to give the SQL Server service account the "Lock Pages in Memory" right and to limit the max memory that SQL Server can take for itself (reserving 1-2GB of RAM for the OS). You also want to use a processor with as much L2/L3 cache as possible with 64 bit. I measured a 45% increase in performance moving from a quad processor Paxille dual-core to a quad processor Tulsa dual-core set up on an otherwise identical system.
Nearly any server processor made in the last two-to-three years supports 64-bit operation, and there is no cost difference between 32 bit and 64 bit versions of Windows and SQL Server 2005. I have not seen any difficulty with 64 bit drivers lately either.
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Glenn Berry's SQL Server Performance 
The link to the MS post is apparently not a direct link to the post. Can you give the post title so we can search for it? Thanks for your help.